Based on the analysis of the graph, it can be observed that among the structures with two adsorption methods, the ones with lowest two-step hydrogenation energy barrier have been chosen for further analysis. The remaining nine structures, indicated by positive free energy changes within the black lines in the graph, need to overcome a certain potential barrier in the first step of hydrogenation. However, six out of these nine structures show relatively negative free energy changes in the last step of hydrogenation indicating that their potential energy determining step may occur in the first hydrogenation step. On the other hand, VB, NbB and NiN exhibit higher energy barriers for the last hydrogenation step indicating that their potential determining steps are more likely to occur in this last step.
Among these structures, CuB has been excluded due to competition reaction of H atom in its first hydrogenation step leading to nitrogen desorption. The remaining structures have been compared based on their respective energy barriers for both steps of hydrogenation. Five structures (NbB, MnB, CoB, CoN and YB) showed larger energy barriers than 0.98 eV while four (VB, FeB, NiN and ReN) exhibited lower energy barriers ranging from 0.31 eV to 0.84 eV and showed relatively good NRR catalytic performance.
Moreover, as shown in Figure 2c and compared with other related studies including several common MOFs photocatalyst and other SACs in BN nanosheet; TM@p-BN (TM = VB, FeB, NiN, ReN) exhibit satisfying low free energy increase in both steps of hydrogenation process which makes them promising candidates for NRR catalytic applications.
Therefore, future research will focus on these four structures: VB@p-BN, FeB@p-BN, NiN@p-BN and ReN@p-BN for detailed investigations and further development.
